This invention relates generally to decontamination of radio-contaminated metals, and in particular to decontamination of radio-contaminated metals by alkaline electrochemical processing. Of particular interest to the present invention is the remediation of radio-contaminated nickel from decommissioning uranium gas diffusion cascades in which nickel is the primary constituent and technetium, neptunium, plutonium and uranium are the primary radioactive contaminants.
The radiochemical decontamination art is presented with unique practical problems not shared with traditional extraction technologies. For example, the presence of only residual parts per million concentrations of reactor fission daughter products such as technetium, neptunium, plutonium, and any other actinides, in remediated nickel and other like recycled products will so degrade product quality of remediated products that their release to unregulated non-nuclear markets is prevented. Degraded product must then either be employed in less valuable regulated nuclear markets or be reworked at greater financial cost.
Various decontamination processes are known in the art, and specifically for decontamination of nickel. Nickel can be removed by selectively stripping from an acidic solution by electrowinning. See U.S. Pat. No. 3,853,725. Nickel may be removed by liquid-liquid extraction or solvent extraction. See U.S. Pat. Nos. 4,162,296 and 4,196,076. Further, various phosphate type compounds have been used in the removal of nickel. See U.S. Pat. Nos. 4,162,296; 4,624,703; 4,718,996; 4,528,165; and 4,808,034.
It is known that metallic nickel, contaminated with fission products, can be decontaminated to remove any actinides present by direct electrochemical processing based on the differences in reduction potential in the electromotive force (emf) series. Actinides have a significantly higher reduction potential (than hydrogen) relative to nickel; therefore, actinides remain in solution while gaseous hydrogen is evolved by electrolysis of water. Consequently, actinides are normally won from molten salt electrolytes rather than from aqueous electrolytes. See U.S. Pat. No. 3,928,153 and 3,891,741, for example. Other electrolytic processes are disclosed by U.S. Pat. Nos. 3,915,828; 4,011,151; 4,146,438; 4,401,532; 4,481,089; 4,537,666; 4,615,776 and 4,792,385.
FIGS. 1 and 2 depict a conventional single electrowinning dissolution system and an entire plant layout, respectively. In FIG. 1 the contaminated metal first is dissolved in anodic dissolution tank 10. The contaminant-containing solution is then transferred out of the dissolving tank to oxidation tank 12, which is equipped with a dispersion system and optionally a blower 26, where the oxidation potential of the radiocontaminants is adjusted. Next, the solution is transferred from the oxidation tank to tank 14, which is equipped with a gas dispersion system and optionally a blower, where oxidants are removed from solution. Then, the solution is transferred through filter 16 which removes solids, and then through a series of ion exchangers 18,20 which remove the radiocontaminants from solution. Next, the solution is transferred to a series of holding tanks 22,24, and subsequently to an electrowinning plating cell (not shown) where nickel is cathodically plated.
FIG. 2 is a schematic representation of a full scale plant layout utilizing the dissolution system of FIG. 1. A series of dissolving tanks 100 (as described above) are serially connected to each other electrically (heavy lines) and via electrolyte piping (thin lines). The dissolving tanks are also connected to holding tanks 120,122 and electrolyte return tanks 130. The dissolving tanks are driven by power supplies 124. Upon dissolution, the solution is transferred to a series of electrowinning plating tanks 126, which are serially connected to power supplies 128, for cathodic deposition of nickel. Thus, while such electrowinning methods are generally effective, they are disadvantageous in that require the use of separate, multiple tanks and power supplies, with a concomitant high cost in capital and operating expense.
In addition, while the removal of uranium and other actinides has been generally addressed by electrowinning or electrorefining techniques, the removal of technetium has continued to be a problem. For example, when nickel is refined by standard electrorefining art in a sulfate electrolyte solution, the technetium had been found to track the nickel and codeposit on the cell cathode. Thus, e.g., experiments employing aqueous sulfuric acid solutions at a pH of about 1-4 at room temperature have shown that the technetium activity of the deposited metal may be as high as the technetium activity of the feedstock. Moreover, such methods require the use of large amounts of resin to remove the technetium. Consequently, a high volume of secondary waste is generated by efforts to remove technetium and other metal contaminants in addition to product recovery. Such problems have been addressed in part in U.S. Pat. Nos. 5,156,722, 5,183,541, and 5,262,019.
What is needed is a method which can effectively recover nickel from nickel contaminated with technetium, other transition metals, and actinide metals, which minimizes equipment requirements, operating costs, and capital costs.
There is a further need for an economical, energy efficient, easy-to-operate method to decontaminate radiocontaminated nickel and, more specifically, to separate technetium, neptunium, plutonium and uranium from nickel, which operates in an environmentally sound manner, and concentrates the contaminated material in a manageable, low volume waste stream.
There is also a need for a method for recovering technetium, other transition metals, and actinide metal contaminants from nickel which reduces the need for secondary processing means to separate/remove such contaminants from nickel, simplifies acid management, and reduces chemical consumption.